FIG. 6 is a schematic exploded perspective view of a conventional image display device. In this conventional image display device, a rear electrode 1, a group of linear cathodes 2 serving as a beam source, a beam extraction electrode 3, a control electrode 4, a focusing electrode 5, a horizontal deflection electrode 6, a vertical deflection electrode 7, and a screen 8 are arranged in this order from the rear towards the anode, and are stored inside a vacuum container (not shown in the drawings).
The group of linear cathodes 2 serving as a beam source is made by extending a plurality of linear cathodes horizontally, so as to generate electron beams that are distributed linearly in a horizontal direction. A plurality of these cathodes is arranged at predetermined intervals in a vertical direction. In this conventional image display device, the intervals between the linear cathodes in the vertical direction are 5.5 mm, with a number of 19 cathodes denoted 2a to 2s. However, in order to avoid making FIG. 6 too complicated, only four linear cathodes from 2a to 2d are shown. The linear cathode 2a to 2s are made by applying an oxide cathode material to the surface of a tungsten wire with a diameter of 10-30 .mu.m, for example. These linear cathodes are then operated for a constant time in sequence from the upper linear cathode 2a to the lower linear cathode 2s, so that each cathode emits an electron beam every 18 horizontal scanning periods.
In addition to prevent the generation of electron beams from linear cathodes other than predetermined linear cathodes, the rear electrode 1 also has the function to ensure that electron beams are emitted only in the direction of the anode. The vacuum container is not shown in FIG. 6, but depending on the circumstances, the rear electrode 1 can be taken and formed in one piece with the vacuum container.
The beam extraction electrode 3 is made of a conductive board 11 provided with a plurality of through holes 10 and has the function to divide and select a plurality of electron beams emitted from the group 2 of linear cathodes horizontally via the through holes 10. On the beam extraction electrode 3, the through holes 10 are arranged on the conductive board 11 with constant horizontal pitch, in opposition to the linear cathodes 2a to 2s. In this conventional image display device, the horizontal pitch of the through holes 10 is 1.28 mm and there are 107 through holes 10 in the horizontal direction.
The control electrode 4 is made of 107 long vertical conductive boards 15, which have through holes 14 that are positioned in opposition to the through holes 10 of the beam extraction electrode 3. However, in order to avoid making FIG. 6 too complicated, only nine conductive boards 15 are shown. Furthermore, based on the image signal for each section, the control electrode 4 simultaneously modulates the throughput of the electron beams that are divided into 107 horizontal sections.
Each section is divided into two pixels, and as each pixel has three primary colors (phosphors) of R (red), G (green), and B (blue), the six signals of 2 (pixels).times.3 (primary colors) that correspond to each section are synchronized with the horizontal deflection, described later, and are then added one after another in time division (within one horizontal scanning period).
The focusing electrode 5 is made of a conductive board 17 that has a plurality of through holes 16, which have the function to focus the electron beam. The through holes 16 in this conductive board 17 are formed in positions opposing the through holes 14 formed in the control electrode 4.
The horizontal deflection electrode 6 is made of a pair of a comb-shaped conductive boards 18 and 18' arranged vertically along both horizontal sides of the through holes 16 formed in the focusing electrode 5, and its function is to simultaneously deflect the electron beam that is divided into 107 sections in a horizontal direction, so that the two groups of the primary color phosphor stripes R, G, and B on the screen 8, which will be described later, are irradiated successively in six stages and emit light.
The vertical deflection electrode 7 is made of a pair of comb-shaped conductive boards 19 and 19' arranged horizontally, in the space between vertically neighboring through holes 16 formed on the focusing electrode 5. With these two conductive boards 19 and 19', the voltage for vertical deflection is applied, and the vertical deflection electrode 7 deflects the electron beam vertically. Here, the vertical deflection electrode 7 deflects the electron beam which is emitted by the 19 linear cathodes 2a to 2s in 12 stages each, or in other words, 12 horizontal scanning line segments each, and 228 horizontal scanning lines are drawn in a vertical direction on the screen 8.
In this conventional image display device as described above, the horizontal deflection electrode 6 and the vertical deflection electrode 7 are both comb-shaped and spread out. Since the distance to the screen 8 is longer than the distance between the horizontal and vertical deflection electrodes, the electron beam can be irradiated onto arbitrary positions of the screen 8 with small amounts of deflection. Therefore, with this configuration, it is possible to decrease the distortion for both horizontal and vertical deflection.
The screen 8 is made of a glass pane, and the R,G and B primary color phosphors which emit light due to irradiation with the electron beam are applied in a stripe-like shape separated by black guard bands (black matrix) onto the rear side of this glass pane, with a metal backing arranged on top (not shown in the drawings). In FIG. 6, the broken lines drawn on the screen 8 show the vertical sections, displayed in correspondence to the plurality of linear cathodes 2a to 2s. Further, the alternate-long-and-two-short dash lines indicate the horizontal sections displayed in correspondence to the plurality of conductive boards 15, which make up the control electrode 4.
In one section, partitioned by both (the broken lines and the alternate long and two short dash lines), as shown enlarged in FIG. 7, two groups of primary color phosphor stripes 20 R, 20 G and 20 B of R, G and B are applied vertically in stripe-like shapes, separated by black guard bands 22 in the horizontal direction. The horizontal lines are formed for 12 lines in the vertical direction. The size of one section (1 unit) in this conventional example is 1.0 mm horizontally, and 4.4 mm vertically, but for illustrative reasons, the lengthwise and crosswise proportions in FIG. 7 are different from the image that appears on the actual screen.
However, in this conventional image display device above, due to thermal expansion of the structural elements when operating the image display device, misalignments between the screen and a group of flat electrodes occur, and due to environmental magnetic fields (for example the earth's magnetism at that position) deviations of the beam track between the group of flat electrodes and the screen occur, causing misalignments between the beam spot and the phosphor stripes on the screen, resulting in a deterioration of the image quality, such as color misalignments, etc.